If you are concerned about the use of aikido or any other response to violence, then it would be worthwhile to check on the laws of self-defense in your jurisdiction. Here in California the criminal statutes allow the use of "reasonable force" to defend yourself or others from violence. Reasonable force is essentially defined as the amount or type of force that a reasonable person would use under the circumstances. There is no specific reference to the use of martial arts or even weapons. The first cops on the scene will certainly look at all the circumstances and may, or may not, make an arrest. There is no requirement here that one retreat. When the cops and prosecutors look at the circumstances, they look at the totality of the circumstances. Time, place, location, actions of the various parties, size, age, gender relationships all play into the totality metrics. A young and healthy six foot male would find himself in deep trouble if he responded to a five foot woman's slap with a pool cue for example, while the reverse situation might be viewed as reasonable.

Anything from a firearm to a flat iron to the use of martial arts and boxing can be considered to be use of deadly force depending on the circumstances. One of the major keys used to make the determination is how much force, how long, and when it was applied. Once the aggressor is neutralized, any further force is probably becomes illegal. I've seen one case where a boxer was charged with an assault with a deadly weapon for continuing to beat on his attacker after the attacker was down and out of the fight. I've seen two other cases where boxers were justified in beating one knife-wielding attacker into a coma and killing another (a single blow to the chest) when being repeatedly attacked with a full wine bottle. There are no simple rules to go by; if he does that then I can do this. You can do whatever is reasonable to protect yourself.

The earlier advice to wait for your lawyer before talking to the police is both good and bad. If you elect to do so, you will very likely be detained until your side of the story is heard. If you tell your side initially, you may avoid even a detention, or you can talk yourself into serious legal consequences. After reading Miranda warnings to hundreds, if not thousands, of people, I know I haven't got anything to say until my attorney arrives - but that is a personal choice and may not be right for you.

By the way, not all homicides are crimes anywhere. Homicide is merely the killing of a human being by another. Murder and manslaughter are crimes which happen to be homicides in which the killing is unlawful.

To paraphrase a current television commercial, "I'm not a lawyer, but I did decide whether you got to spend the night in jail or at a Holiday Inn."